It has been known to form a thin piece portion for a TEM observation or the like, by performing an etching process on a sample with a focused ion beam (FIB) system (for example, see JP-A-2003-194681). The thin piece portion is formed by removing both sides of a region to be a thin piece portion by irradiating a sample with an FIB from above the sample and by sequentially forming observation surfaces parallel to the irradiation direction of the FIB.
In an etching process called a lift-out method, after a rough process which forms an outline by removing both sides of a region to be a thin piece portion by irradiating a sample with an FIB while increasing a beam current, a cut is formed by inclining a sample surface and irradiating a bottom surface of the thin piece portion with the FIB. Next, a finishing process is performed on an observation surface by returning the sample surface to an original state, and irradiating the sample from above the sample with the FIB while reducing a beam current, and is terminated at a state where the interval between the observation surfaces reaches a predetermined value so as to achieve the thin piece portion having a predetermined thickness. Finally, a cut is formed by irradiating both sides of the thin piece portion with the FIB, and the thin piece portion is separated from the sample so as to be used appropriately in a TEM observation or the like.
When the sample is processed with the FIB, since there is an intensity distribution in a lateral cross-sectional direction (a direction perpendicular to the irradiation direction) in the FIB, even if the sample is irradiated with the FIB perpendicular to the sample surface, the corner portion of the sample is etched and thus an accurate cross-sectional shape cannot be obtained. Thus, a protective film made from a metal film or a carbon film is formed on the surface of the sample to prevent etching in the lateral direction by the FIB.
In recent years, as an object to be observed by a TEM becomes smaller, the thickness of a thin piece portion becomes thinner, for example, the thickness is required to be several tens of nm or less (for example 50 nm or less). Further, since work saving and sample preparing operations which do not require skill are demanded, it is necessary to automatically prepare a TEM sample in an FIB system.
However, the thinner the thickness of the thin piece portion becomes, the longer the time for the finishing process is required, so that a time of a protective film being exposed to the FIB becomes longer. Therefore, the protective film may be cut and entirely removed during the process. If the protective film is entirely removed, the processing speed of the sample (removal speed of the sample) rapidly increases and the thin piece portion is excessively cut, so that it is difficult to control the thin piece portion so as to have a target thickness, or a cross-sectional shape becomes inaccurate due to the intensity distribution in the lateral cross-sectional direction of the beam as described above.
Incidentally, the thickness of the thin piece portion is directly measured from an intensity change (decrease rate of contrast) of a back-scattered electron signal by irradiating, with electron beams, the thin piece portion being processed. However, in this case, a relationship between a film thickness and a contrast is required to be obtained in advance for each type of sample, and thus it takes time for pre-preparation. Further, since the contrast varies depending on the state of the observation surface, the irradiation condition of the electronic beam, the thickness of the thin piece portion, or the like, it is difficult to measure the thickness of the thin piece portion with high accuracy.
Thus, as the thickness of the thin piece portion becomes thinner, it is difficult to identify an end point of the removal process.